1. Field of the Invention
The present invention relates to a lens system for 35 mm lens shutter, and more particularly to a compact and low-cost lens system.
2. Related Background Art
The lens systems for lens shutter cameras are recently decreasing in size while increasing the zoom ratio more and more. The simplest type of lens system, among a variety of zoom types suitable for such lens systems, is a telephoto type lens system composed of two units, a first lens unit G.sub.1 having a positive refractive power and a second lens unit G.sub.2 having a negative refractive power. This zoom type is suitable for compact zoom lenses with some short back focus, and an example of this type is disclosed in Japanese Patent Application Laid-Open No. 3-200912.
Further, an example of a low-cost lens system is disclosed in Japanese Patent Application Laid-Open No. 3-127012, which discloses a lens system employing aspherical surfaces to reduce the number of constituent lenses. In addition, Japanese Patent Application Laid-Open No. 3-200913 discloses a lens system employing a plastic lens.
In the lens system of Japanese Patent Application Laid-Open No. 3-200912, however, the first lens unit G.sub.1 was constructed in an arrangement of positive, negative, and positive refractive powers, in which a component of the negative refractive power had a lens with a strong concave surface facing the object in order to correct a negative spherical aberration and a positive distortion. This lens surface was in turn a convex surface facing the stop, which made corrections of a lower coma difficult, thus causing large aberrations of lower chromatic coma.
In the lens system of Japanese Patent Application Laid-Open No. 3-127012, the first lens unit G.sub.1 was constructed in an arrangement of negative and positive refractive powers and correction of aberrations was effected by using many aspherical surfaces. Off-axis aberrations were substantially improved for monochromatic light in the lens system of the Japanese Patent Application Laid-Open No. 3-127012, but correction of transverse chromatic aberration was not sufficient because the number of constituent lenses was small, thus resulting in insufficient achromatism. Further, lens thickness of each unit (a distance between the most object-side surface and the most image-side surface) was not able to be set small because of low degrees of freedom on correction of aberrations, which was not preferable with respect to the size reduction and weight reduction of the camera. In addition, there was a large gap between a negative lens component and a positive lens component constituting the first lens unit, which caused considerable differences of astigmatism depending upon colors.
Further, in the lens system of Japanese Patent Application Laid-Open No. 3-200913, the first lens unit G.sub.1 was constructed in an arrangement of negative and positive refractive powers, in which a concave lens was made of a low-dispersive glass material and, therefore, a convex component needed to be used for excessive achromatism, which increased the number of lenses in the first lens unit G.sub.1, thus did not reduce the cost.